Method of treating steel and a novel steel product



Sept. 26, 1967 M. BALDY ETAI- 3,344,000

METHOD OF TREATING STEEL AND A NOVEL STEEL PRODUCT Filed May 20, 1965 MAURICE BALD) and DAN/EL r sous/mm Gum,

United States Patent ()ffice 3,344,000 Patented Sept. 26, 1967 3,344,000 METHOD OF TREATING STEEL AND A NOVEL STEEL PRODUCT Maurice F. Baldy, Penn Hills Township, Allegheny County, and Daniel T. Boughner, West Mifliin Borough, Pa.,

assignors to United States Steel Corporation, a corporation of Delaware Filed May 20, 1965, Ser. No. 457,321 5 Claims. (Cl. 148-123) ABSTRACT OF THE DISCLOSURE A method of treating steel of a particular composition to improve its high temperature properties which comprises normalizing by heating the steel at a temperature of about 1800 to 2100 F. to dissolve substantially all carbides and then air cooling, cold working to a reduction of from about 5 to about 25% and tempering in the range of from about 1225 to 1400 F. to agglomerate complex carbides precipitated in the grain boundaries. Also dis closed is a novel steel product produced by the aforementioned method having (a) agglomerated intergranular and intragranular carbides randomly distributed therein; (b) a minimum ductility at 1100 F. is measured in a creep rupture test of in l-inch and reduction in cross sectional area; and (c) a rupture strength of at least 11,000 p.s.i. after 10,000 hours at 1100 F.

The present invention involves a method of improving properties of steel exposed to high temperatures and to a novel steel product. More particularly, the invention involves a method of treating steel which has good hightemperature strength properties even after prolonged exposure to elevated temperatures, so that the steels ductility may be maintained and high-temperature embrittlement is avoided. Also, a part of the invention is a novel form of steel which possesses the aforementioned desirable properties.

Among steels which are recommended for high-temperature applications, such as for use in boiler plate, boiler tubes, pressure vessels, etc., are those which contain chromium, molybdenum and vanadium in controlled quantities. Such steels possess desired high-strength properties at elevated temperatures but have shown undesirable embrittlement upon prolonged exposure to elevated temperatures. The invention provides a method of treating these steels which preserves the steels desirable highstrength while rendering it ductile and not susceptible to embrittlement after prolonged exposure to high temperature.

The steel compositions which have good high-temperature strength and whose ductility upon prolonged exposure to elevated temperature is improved by the invention are nominally 1% chromium, 1% molybdenum and 0.2% vanadium steels. Such steels may'have in percent by weight up to about 0.3% carbon, 0.25 to 0.75% manganese, 0.5 to 1.5% chromium, 0.2 to 1.25% molybdenum, 0.07 to 0.4% vanadium, 0.05 to 0.4% silicon, up to 0.045% phosphorus and up to 0.045 sulfur. The preferred limits are 0.3 to 0.6% manganese, up to about 0.2% carbon, 0.05 to 0.35% silicon, 0.8 to 1.25% chromium, 0.8 to 1.15% molybdenum, 0.15 to 0.25% vanadium, up to 0.03% phosphorus, and up to 0.03% sulfur. The high strength of this steel is developed by a high-temperature normalizing treatment which dissolves complex carbides and, in particular, vanadium carbides, so as to yield a finely dispersed precipitate upon cooling and subsequent tempering. However, exposure to elevated temperature for even a relatively short time causes formation of a continuous grain boundary carbide film which is responsible for the extreme embrittlement of the steel. The high-temperature embrittlement has obviously severely limited the use of this composition in high-temperature applications.

It has been observed through metallographic examination of creep-rupture specimens that the embrittlement described above occurs concurrently with the formation of the aforementioned continuous grain boundary carbide network. We have found a practical and economical method of spheroidizing the grain boundary carbide network so that the resulting product has excellent ductility and is free from intergranular embrittlement upon prolonged exposure at high temperature. According to the invention, steels of the aforementioned composition are first normalized then cold Worked to a sufiicient reduction and finally tempered.

In practicing our invention, steel is normalized in a conventional manner at a relatively high temperature in the range from about 1800 to 2100 F. for a time suificient to dissolve all carbides. This usually requires heating in this temperature range for one hour per inch of thickness with a two-hour minimum. As part of the normalizing treatment, the steel at the end of the soaking or holding time is allowed to cool in air. Normalizing treatments tend to produce a steel with higher yield and tensile strengths than annealed products. After normalizing, the steel is cold worked according to the invention to a reduction of at least 5% and generally of from 5 to 25 After cold working, the steel is tempered in the range of from about 1225 to about 1400 F. to precipitate the complex carbides and agglomerate those at the grain boundary. The product, after tempering, is characterized by agglomerated intergranular and intragranular carbides randomly distributed throughout the steel.

Our procedure has the particular economical advantage of being able to be performed in a reasonably short time. It is important, however, that the aforementioned limits be maintained. For example, it is necessary to is exposed, the greater the degree of scale formation, and

consequently, the less economical will be the practice.

Although it is not entirely certain, it is believed that the dislocations and misalignment within the grains resulting from cold working act as preferred nucleation sites for the carbide precipitation during the subsequent tempering. It is also believed that increasing the amount of cold Work increases the number of nucleation sites, thereby speeding up the carbide agglomeration during tempering and requiring less time for the agglomeration or spheroidizing process to occur. If much less than 5% cold work is performed, the time required for spheroidizing during tempering is unduly increased and renders the procedure impractical from an economical point of view. More'than about 20% cold work, however, tends to decrease the strength properties of the steel, although steel having high-temperature strength satisfactory for many purposes and with exceptionally good ductility can be produced with more than 20% cold reduction, e.g. 2 0 to 25%. Cold working to about a 10 to 20% reduction is preferred as providing the presently considered best combination of high-temperature strength and ductility upon prolonged exposure to elevated temperatures.

During the tempering step, the carbides precipitated at the grain boundary as a fine continuous layer are spheroidized, i.e. agglomerated, thereby breaking up the continuous grain boundary carbide layer into a discontinuous carbide phase. For economic reasons, the spheroidizing process should be accomplished in as reasonably short a time period as possible.

In the tempering range of from about 1225 to 1400' F., the spheroidizing can be completed in from about to 20 hours. At the preferred temperature range of 1300 to 1375 F., 8 to 17 hours may be sufiicient. If a tempering temperature less than about 1225 F. is used, a disproportionately longer time period will be required for breaking up the grain boundary carbide network. Above about 1400 F., a phase change would occur in the steel which should be avoided. The overall preferred procedure involves normalizing at a temperature of 1850 to 1950 F. for at least two hours, cold working to a reduction of from to and tempering in the range of from 1300 to 1375 F. for from about 8 to 17 hours.

Conventionally, steel articles for high temperature uses such as boiler tubes, boiler plate, etc., are made by cold Working before normalizing. We have found that, as explained above, the embrittlement is caused by the continuous grain boundary film of carbides that precipitate during use or exposure to elevated temperature. By cold working the steel after normalizing and then tempering, the grain boundary carbides can be spheroidized prior to high-temperature exposure rather than after the steel article is placed in use. In the past, substantially all the cold working has been elfected prior to heat treatment except possibly, for a slight sizing pass after annealing. A cold working operation between normalizing and tempering may now replace all or a portion of the cold working conventionally performed prior to heat treating. If cold working prior to heat treating is not required in a particular operation, workpiece sizing may be compensated for during prior hot Working. In tube manufacture, cold working is in the form of cold drawing by pulling a tube through a die, the opening of which is smaller than the outside diameter of the tube being drawn, and at the same time the inside surface of the tube may or may not be supported on a mandrel anchored at one end. However, any convenient cold working operation may, of course, be employed as long as the required amount of cold Work is imparted to the workpiece and the prior normalizing and subsequent tempering conditions set forth herein are observed.

The following illustrations will be helpful in understanding the invention. These examples are summarized in Table I. The steel used in all examples had the following composition:

C, 0.13; Mn, 0.40; P, 0.010; S, 0.015; Si, 0.12; Cr, 1.18; Mo, 0.98; V, 0.21; N, 0.007.

A series of specimens /:.-inch square by %-inch thick cut from bar stock were treated as hereinafter described in accordance with conventional processing. The first group of samples was tempered at 1300 F. for 12 hours after normalizing at 1925 F. for 2 hours. This procedure, designated as treatment A, may be regarded as representative of conventional processing. A second set of samples also representative of conventional processing was normalized at 1925 F. for 2 hours and tempered at 1350 F. for 12 hours. The latter processing is designated as treatment B.

In accordance with our presently preferred practice of the invention, a series of samples of the above composition were prepared from %-inch square of 10-inch lengths by normalizing at 1925 F. for 2 hours then machining into rods of circular section. The rods were then pulled through a die, the area of which was 15% less than the cross section of the rod, and then samples were cut from the cold reduced rod. One set of samples was tempered for 10 hours at 1350 F. and another set was tempered for 15 hours at 1350 F. These procedures are designated treatments C and D, respectively. To demonstrate the efiect of large cold reductions, some samples were cold reduced 4 21.5% after normalizing and then tempered at 1350 F. for 10 or 15 hours. The latter treatments are designated as treatments E and F.

Table I summarizes the various treatments discussed above and Table II and III report the results of a series of tests to determine ductility after varying periods of exposure to temperatures of 1000 F. and higher which were performed on the specimens treated according to the different techniques described in Table I. It should be particularly noted that the ductility, indicated by elongation characteristics, is greatly improved in the specimens treated according to the invention.

TABLE I Normalizing Condition Tempering Condition Treatment Identity Temp, Time, Percent Temp., Time,

F. Hr. Cold F. Hr.

Draw

TABLE II Time To Total Percent Treatment ldentlty Stress, Rupture, Elong, Red. or

1,000 p.s.i. Hrs. Percent Area It is shown by the data in Table II that products of treatments A and B exhibit a progressive loss in ductility as time at temperature increases. In contrast, the samples given treatments C and D maintain a satisfactory level of ductility after prolonged exposures to elevated temperatures. The improvement in ductility of the samples given treatments C and D reflects the interruption of the continuous grain boundary carbide network. As discussed above, cold working is believed to increase the number of nucleation sites and thereby increases and speeds up spheroidization of the carbide phase. The data in both tables point out that large cold reductions increase the ductility considerably while reducing the strength. Thus, where extremely high ductility is desired, greater cold reduction should be performed. It should be noted that although the strength is reduced as shown, this strength level at elevated temperature is still considerably better than competitive steels.

The effect of cold work on this steel is further shown in the following examples. Four blanks %-inch square and 10 inches long were cut from bar stock of the aforementioned composition and were normalized at 1925 F. for 2 hours. After normalizing, the blanks were machined into rods which were cold worked 5, 10, 15 and 20% by cold drawing through an 0.4565-inch diameter die. After cold drawing, the rods were cut to /z-inch lengths and tempered at 1300 F. for hourly periods varying from 1, 2, 4, 8 and up to 128 hours. Examination of these samples showed the following:

cold draw.-The grain boundary carbide network was continuous after 4 hours at 1300 F. It became completely discontinuous between 32 and 64 hours. There was no evidence of any recrystallization.

10% cold draw.The grain boundary carbide network was continuous after 1 hour at 1300 F. It was fairly well broken up after 8 hours at 1300 F. and was essentially completely discontinuous after 16 hours at 1300" F. There were some indications of the beginnings of recrystallization after 128 hours at 1300 F.

cold draw.The grain boundary carbide network had begun to break up in the 1 hour sample and was completely discontinuous in 4 hours although the average size of the carbides was very fine. These carbide particles grew with increasing time and reached a larger size with very even distribution between 8 and 16 hours at 1300 F. The sample exposed at 1300 F. for 32 hours showed the beginnings of recrystallization which continued and was complete in 128 hours.

20% cold draw.The structural changes were the same as the 15% cold draw except recrystallization began in 16 hours at 1300 F. and was essentially complete in 64 hours.

In the drawing are shown two photomicrographs 1abeled FIGURES 1 and 2 which serve to illustrate the unique metallurgical article of the invention. A typical product of conventional processing by normalizing a steel of the above composition at 1925 F. for two hours and tempering at 1300 F. for 12 hours is shown in FIGURE 1 at 1000 times magnification after a nital picral etch. FIGURE 2 is a photomicrograph of a specimen of the same composition normalized at 1925 F. for two hours, cold drawn 15% and tempered 15 hours at 1350 F. The FIGURE 2 specimen has a metallurgical structure consisting of very fine carbides (with a considerably fine-r average particle size than the conventional product) randomly distributed throughout. Moreover, as is evident, the carbides in the structure shown in FIGURE 2 are agglomerated, i.e., spheroidized, both within the grains and along the grain boundaries.

It is apparent from the above that various changes or modifications can be made without departing from the invention. The desirable discontinuous grain boundary carbide network can be achieved with short tempering times if the material is cold worked after normalizing and before tempering. It is further noted that by confining the cold reduction within the range of 10 to 15 the relatively long tempering periods necessary with the 5% reduction can be avoided. Moreover, because a reduction in strength occurs with recrystallization, it is preferred to apply less than the maximum cold reduction, since as noted, recrystallization occurs within a relatively short time when using a 20% or greater cold draw. In contrast, the conventionally processed specimen contains large carbide particles locally concentrated and not randomly distributed. The carbides are present in feathery or streaky patterns and, generally, as a continuous network along the grain boundaries. Upon prolonged exposure at elevated temperature, the carbide precipitation at the grain boundary intensifies until the steel loses its ductility and becomes extremely brittle. In contrast, steels with micro structures such as shown in FIGURE 2 will possess good ductility while retaining high strength when subjected to high temperatures over extended periods. Thus, for example, shown in Table II, samples processed according to the invention possessed a minimum ductility, on the average, of 10% enlongation in l-inch and 20% reduction in cross-sectional area while retaining a rupture strength of at least about 11,000 p.s.i. after 10,000 hours at 1100 F.

We claim:

1. A method of improving the high-temperature properties of a steel having in percent by weight up to 0.3% carbon, 0.25 to 0.75% manganese, 0.5 to 1.5% chromium, 0.2 to 1.25% molybdenum, 0.07 to 0.4% vanadium, up to 0.4% silicon, up to 0.045% phosphorus and up to 0.045% sulfur comprising normalizing by heating said steel to a temperature of about 1800 to 2100" F. for a time suflicient to dissolve substantially all carbides and then air cooling, cold working to a reduction of from about 5 to about 25% and tempering in the range of from about 1225 to about 1400 F. to agglomerate complex carbides precipitated at the grain boundaries.

2. A method of improving the high-temperature properties of a steel having in percent by weight up to 0.3% carbon, 0.25 to 0.75% manganese, 0.5 to 1.5% chromium, 0.2 to 1.25% molybdenum, 0.07 to 0.4% vanadium, 0.05 to 0.4% silicon, up to 0.045% phosphorus and up to 0.045% sulfur comp-rising normalizing by heating said steel to a temperature of about 1800 to 2100 F. for at least about 2 hours and then air cooling, cold working to a reduction of from about 10 to about 20% and tempering in the range of from about 1225 to about 1400 F. for about 5 to 20 hours to agglomerate complex carbides precipitated at the grain boundaries.

3. A method of improving the high-temperature properties of a steel having in percent by weight up to 0.2% carbon, 0.3 to 0.6% manganese, 0.8 to 1.25% chromium, 0.8 to 1.15% molybdenum, 0.15 to 0.25% vanadium, 0.05 to 0.35% silicon, up to 0.3% phosphorus and up to 0.03% sulfur comprising normalizing by heating said steel to a temperature of about 1850 to 1950 F. for at least about 2 hours to dissolve substantially all carbides and then air cooling, cold working to a reduction of about 10 to 15% and tempering in the range of from about 7 1300 to about 1375 F. for about 8 to 17 hours to agglomerate complex carbides precipitated at the grain boundaries.

4. A steel product consisting essentially of, in percent by Weight, up to 0.3% carbon, 0.25 to 0.75% manganese, 0.5 to 1.5% chromium, 0.2 to 1.25% molybdenum, 0.07 to 0.4% vanadium, 0.05 to 0.4% silicon, up to 0.045% phosphorus and up to 0.045% sulfur, said steel having (a) agglomerated intergranular and intragranular carbides randomly distributed therein, (b) a minimum ductility at 1100 F. as measured in a creep rupture test of 10% elongation in l-inch and 20% reduction in crosssectional area and (c) a rupture strength of at least 11,000 p.s.i. after 10,000 hours at 1100 F.

5. A steel product according to claim 4 consisting essentially of, in percent by weight, up to about 0.2% carbon, 0.3 to 0.6% manganese, 0.05 to 0.35% silicon, 0.8 to 1.25% chromium, 0.8 to 1.15% molybdenum, 0.15 to 0.25% vanadium, up to 0.03% phosphorus and up to 0.03% sulfur.

References Cited UNITED STATES PATENTS 3,291,655 12/1966 Gill et a1. 14836 HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

H. F. SAITO, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,344,000 September 26, 1967 Maurice F. Baldy et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 70, for "0.3% phosphorus" read 0.03% phosphorus Signed and sealed this 22nd day of October 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A METHOD OF IMPROVING THE HIGH-TEMPERATURE PROPERTIES OF A STEEL HAVING IN PERCENT BY WEIGHT UP TO 0.3% CARBON, 0.25 TO 0.75% MANGANESE, 0.5 TO 1.5% CHROMIUM, 0.2 TO 1.25% MOLYBDENUM, 0.07 TO 0.4% VANADIUM, UP TO 0.4% SILICON, UP TO 0.045% PHOSPHORUS AND UP TO 0.045% SULFUR COMPRISING NORMALIZING BY HEATING SAID STEEL TO A TEMPERATURE OF ABOUT 1800 TO 2100*F. FOR A TIME SUFFICIENT TO DISSOLVE SUBSTANTIALLY ALL CARBIDES AND THEN AIR COOLING, COLD WORKING TO A REDUCTION OF FROM ABOUT 5 TO ABOUT 25% AND TEMPERING IN THE RANGE OF 